We humans are social animals. Most of us interact with others on a daily basis, and form complex social groups containing dozens of friends, relatives and peers. Maintaining such a network involves tracking our own relationships to individuals within it, and also requires some understanding of their relationships to one another.

With such knowledge, we can quickly figure out another person's social standing and, with that, infer how best to behave toward them. But what happens in the brain during this process? Research published today in Nature Human Behavior shows seeing familiar people activates a network of brain regions that appears to encode their position within the social group.

Carolyn Parkinson of the University of California, Los Angeles, and her colleagues from Dartmouth College recruited 275 first-year MBA students and asked them to complete an online survey that included questions about which classmates they like to spend their free time with, who they visit at home and who they have been with most often for informal social events such as going to lunch, dinner, drinks or to the movies.

They used the survey responses to assess each participants’ position within the social network of their academic program, according to three different measures: their social distance, or “degrees of separation” from one another; their proximity to other, well-connected individuals in the network; and the extent to which they connected otherwise unconnected individuals in the network. The researchers then used functional magnetic resonance imaging (fMRI) to scan 21 of the participants’ brains while they viewed pairs of short film clips showing classmates of varying status within this social network, telling them all they needed to do was indicate whether the clips in each pair were the same or different, and that this task was unrelated to the first part of the experiment.

Previous research has already identified components of the brain’s social network, revealing the inferior parietal lobule seems to encode familiarity and social distance, and that the medial prefrontal cortex encodes how socially relevant other people are to us. In keeping with these earlier findings, Parkinson and her colleagues found activity in a widely distributed network of brain regions was sensitive to the social status of the people in the film clips, with individual regions responding to different aspects of it. Prefrontal cortex activity, for example, increased according to how well connected each person was whereas the inferior parietal lobule was particularly sensitive to the social distance between the viewer and the person being viewed. “Whenever we see another person, our brains spontaneously register a wealth of information about them, including things like their gender, personality traits, emotions and so on,” Parkinson says. The results suggest that “we also spontaneously activate knowledge of where they sit in our social networks to prepare us to think about and interact with them appropriately” she adds.

“This looks like an excellent study,” says psychiatrist Leonhard Schilbach, group leader of the Independent Max Planck Research Group for Social Neuroscience who was not involved with the study. “What I like most is the idea of combining information about everyday-life social networks with standardized brain imaging to assess implicit measures of social perception [that are relevant to the real world].”

Another observer not associated with the work—Antonia Hamilton, director of the Social Neuroscience Group at University College London—echoes this view: “The novelty lies in getting a real-world measure of distance and social connections.” She adds a caveat, however: “My only worry is that social distance is strongly correlated with familiarity—you might get similar results if the brain just encodes people you see more often in more detail.”

But Parkinson says she and her colleagues took this into account and factored it into their experimental design. “It is, of course, the case that we tend to have more intimate relationships with our friends than with familiar acquaintances,” she says, “and that comes out in the participants’ ratings of how close their relationships were with each person they viewed in the scanner.” She adds, "One way we controlled for this familiarity was that participants never saw strangers in the scanner. Some individuals were their friends, some were friends of their friends and some were friends of their friends’ friends (two vs. three degrees distant). The brain regions that appear to encode social distance responded quite differently to people who were two and three "degrees away" from participants in the social network, although their own ratings of closeness to people in those two categories were quite similar.”

Parkinson says she now wants to understand how this social information might influence people's behavior. “A natural question that follows from these findings is, ‘How is this social network position information used?’” she says, “and we’re currently working on behavioral studies investigating how knowledge of others’ social status shapes things like attention, empathy and trust.”

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